Often, a microwave filter in a cellular telephone base station is required to transmit only a certain fraction of the bandwidth for a given communication system. For example, if the receive bandwidth for a given communication system is 1850-1910 MHz, the microwave filter may be required to transmit only a certain 20 MHz sub-band (i.e. 1870-1890 MHz). Additionally, a given communication system may require the ability to switch or change between different sub-bands. As a result, the filter needs to have the ability to tune to another sub-band. It is desirable for the filter to be adjustable remotely. In other words, it is desirable to be able to adjust or tune the filter to different sub-bands without having to send a technician into the field to manually or mechanically adjust or tune the filter.
Typically, a microwave filter is tuned by adjusting the resonant frequency of the resonator. Currently, the resonators are tuned by using a metal material to selectively disrupt the electromagnetic energy distribution in the resonator. This is typically accomplished by manually or mechanically turning a tuning screw in the resonator. There is typically one tuning screw per resonator, and a plurality of resonators per filter.
However, manually or mechanically turning the tuning screws in the resonator creates a number of problems. First, manually tuning, by definition, cannot be done remotely. This requires a technician to travel to the base station to tune the resonator. Second, mechanically tuning creates mechanical problems because a number of moving parts may be required, such as a motor to turn the screws. The motors are prone to mechanical failure. Third, although mechanically turning screws and thereby adjusting the resonant frequency of the resonator is possible remotely, it is relatively expensive to implement.
Based on the above problems, it is desirable to have a remotely adjustable microwave filter that is reliable, accurate and inexpensive.